1. Field of the Invention
The present invention relates to an optical sheet and, particularly, to a brightness enhancing optical sheet.
2. Description of the Prior Art
In recent years, the traditional cathode ray tube display (commonly called CRT display) is being gradually replaced by a liquid crystal display (LCD). This is mainly because the LCD releases far less radiation than the CRT display, and the production cost of the LCD also dropped significantly in recent years. In general, the LCD consists of two main elements, namely a backlight module and a liquid crystal panel. The backlight module mainly aims to provide light to the LCD.
Refer to FIG. 1 for a conventional backlight module. The backlight module 100 mainly includes a cold cathode fluorescent lamp (CCFL) 110, a reflection box 120, a diffusion plate 130 and a plurality of optical films 140. The CCFL 110 aims to generate light. The reflection box 120 aims to direct the light generated by the CCFL 110 towards the diffusion plate 130. The optical films 140 includes a diffusion film 142 and a brightness-enhancement film (BEF) 144. The diffusion plate 130 aims to diffuse the light generated by the CCFL 110 and project the light to a liquid crystal panel (not shown in the drawings) to make the light more uniform to prevent uneven brightness on the LCD. The diffusion plate 130 contains a plurality of light diffusion particles which lower the transmittance of the diffusion plate 130. In general, the transmittance of the diffusion plate 130 is between 50%-70%.
However, the diffusion plate 130 often cannot fully overcome the problem of uneven brightness. Hence, a diffusion film 142 has to be added to diffuse the light more evenly. Moreover, as the light emission angle of the light emitted from the diffusion film 142 is larger, the BEF 144 has to be added on an upper side of the diffusion film 142. The BEF 144 has a thickness about 0.062 mm to 0.375 mm. The BEF 144 mainly includes a substrate 144a and a plurality of microstructures 144b disposed on the substrate 144a. The microstructures 144b are prism structures in a triangular shape, and each has a cross section in the form of an isosceles right triangle in the vertical direction. The BEF 144 provides a light converging effect and, thus, can enhance the brightness within the visual angle range of the backlight module 100.
Because of manufacturing process and material, the BEF 144 is the most expensive in the cost of the backlight module 100. Referring to FIG. 2, in order to reduce cost, a technique has been developed that forms a plurality of microstructures 130b′ on the diffusion plate 130′. The microstructures 130b′ are triangular struts to provide a light converging effect. Hence the diffusion plate 130′ can replace the BEF 144 and reduce the production cost of the backlight module 100′. However, the light converging effect provided by the diffusion plate 130′ still is not as desirable as the BEF 144. Hence, how to improve the light converging effect of the diffusion plate is an issue remaining to be resolved in the industry.